From: Dave Hansen <dave.hansen@linux.intel.com>

Date: Tue, 21 Feb 2023 12:30:15 -0800

Subject: [PATCH] uaccess: Add speculation barrier to copy_from_user()

References: bsc#1012628

Patch-mainline: 6.2.1

Git-commit: 74e19ef0ff8061ef55957c3abd71614ef0f42f47

commit 74e19ef0ff8061ef55957c3abd71614ef0f42f47 upstream.

The results of "access_ok()" can be mis-speculated.  The result is that

you can end speculatively:

	if (access_ok(from, size))

		// Right here

even for bad from/size combinations.  On first glance, it would be ideal

to just add a speculation barrier to "access_ok()" so that its results

can never be mis-speculated.

But there are lots of system calls just doing access_ok() via

"copy_to_user()" and friends (example: fstat() and friends).  Those are

generally not problematic because they do not _consume_ data from

userspace other than the pointer.  They are also very quick and common

system calls that should not be needlessly slowed down.

"copy_from_user()" on the other hand uses a user-controller pointer and

is frequently followed up with code that might affect caches.  Take

something like this:

	if (!copy_from_user(&kernelvar, uptr, size))

		do_something_with(kernelvar);

If userspace passes in an evil 'uptr' that *actually* points to a kernel

addresses, and then do_something_with() has cache (or other)

side-effects, it could allow userspace to infer kernel data values.

Add a barrier to the common copy_from_user() code to prevent

mis-speculated values which happen after the copy.

Also add a stub for architectures that do not define barrier_nospec().

This makes the macro usable in generic code.

Since the barrier is now usable in generic code, the x86 #ifdef in the

BPF code can also go away.

Reported-by: Jordy Zomer <jordyzomer@google.com>

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Acked-by: Daniel Borkmann <daniel@iogearbox.net>   # BPF bits

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

---

 include/linux/nospec.h | 4 ++++

 kernel/bpf/core.c      | 2 --

 lib/usercopy.c         | 7 +++++++

 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/nospec.h b/include/linux/nospec.h

index c1e79f72..9f0af4f1 100644

--- a/include/linux/nospec.h

+++ b/include/linux/nospec.h

@@ -11,6 +11,10 @@

 struct task_struct;

+#ifndef barrier_nospec

+# define barrier_nospec() do { } while (0)

+#endif

+

 /**

  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise

  * @index: array element index

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c

index ba3fff17..430c66d5 100644

--- a/kernel/bpf/core.c

+++ b/kernel/bpf/core.c

@@ -1910,9 +1910,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)

 		 * reuse preexisting logic from Spectre v1 mitigation that

 		 * happens to produce the required code on x86 for v4 as well.

 		 */

-#ifdef CONFIG_X86

 		barrier_nospec();

-#endif

 		CONT;

 #define LDST(SIZEOP, SIZE)						\

 	STX_MEM_##SIZEOP:						\

diff --git a/lib/usercopy.c b/lib/usercopy.c

index 1505a52f..d29fe29c 100644

--- a/lib/usercopy.c

+++ b/lib/usercopy.c

@@ -3,6 +3,7 @@

 #include <linux/fault-inject-usercopy.h>

 #include <linux/instrumented.h>

 #include <linux/uaccess.h>

+#include <linux/nospec.h>

 /* out-of-line parts */

@@ -12,6 +13,12 @@ unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n

 	unsigned long res = n;

 	might_fault();

 	if (!should_fail_usercopy() && likely(access_ok(from, n))) {

+		/*

+		 * Ensure that bad access_ok() speculation will not

+		 * lead to nasty side effects *after* the copy is

+		 * finished:

+		 */

+		barrier_nospec();

 		instrument_copy_from_user_before(to, from, n);

 		res = raw_copy_from_user(to, from, n);

 		instrument_copy_from_user_after(to, from, n, res);

-- 

2.35.3